An ignition plug used for an internal combustion engine or the like includes, for example, a center electrode extending in an axial direction, an insulator provided around the center electrode, a tubular metallic shell provided around the insulator, and a ground electrode whose proximal end portion is joined to a forward end portion of the metallic shell. Through application of a high voltage to the center electrode, spark discharge is generated at the gap formed between the center electrode and the ground electrode, whereby fuel gas is ignited.
In recent years, there has been proposed an ignition system which is configured to generate high-frequency plasma by supplying a high-frequency current to the spark discharge that is generated at the gap of an ignition plug through application of a high voltage thereto, to thereby enhance ignition performance (see, for example, International Publication No. PCT/JP2009/088045). Such an ignition system includes a discharge power supply for applying a high voltage to the gap, a high-frequency power supply for supplying a high-frequency current to the gap, and a mixer which is connected to the ignition plug through which output currents from the two power sources flow.